RAMAN-STUDY OF THE NH4+ LIBRATION PHONON COUPLING IN NH4MNF3

The temperature dependence of the bandwidth of the librational modes o f the NH4+ rotator on the orthorhombic (low-temperature) structure of the NH4MnF3 perovskite has been obtained from Raman spectra. This depe ndence has (low-temperature) structure of the NH4MnF3 perovskite has b een obtained from Raman spectra. This dependence has been analysed in the framework of libration-phonon coupling giving rise to cubic and qu artic anharmonic terms in the interaction potential of the atoms in th e crystal. Evidence is given that the librational modes at 342 and 355 cm(-1) are coupled to phonon modes which involve rotations of MnF6 oc tahedra and translational motions of the NH4+ group itself.